Abstract
The importance of lakes and reservoirs leads to the high need for monitoring lake water quality both at local and global scales. Remote sensing is a rapidly evolving, versatile technology that can be successfully applied in several economic and scientific fields. Numerous studies demonstrate the applicability of satellites in algae detection and monitoring. Algae play an essential role in aquatic ecosystems, although their overgrowth poses a serious risk. Overgrowth of algae, also known as algal bloom, has serious ecological, social, economic and health effects. The research area of our study was Lake Balaton, the largest lake in Central Europe. The aim was to find the most appropriate algorithm(s) for the inland lake to define the chlorophyll-a amount. In addition, two new algorithms were developed based on the reflectance values of the satellite image. The results show that the two highest correlations were performed by the newly validated, blue/green ratio-based algorithm, Chl-aB/G (r = 0.93) and the chl_re_oc2 algorithm (r = 0.86) of the Acolite software. Although the Acolite software was not developed for lakes but for marine waters, it is also applicable to inland waters.
Highlights
Stored in lakes, wetlands and rivers, surface freshwater is the most significant resource that sustains environment and the lives of the organisms. 87% of inland waters are lakes and reservoirs and cover about 1.9 million k m2 of the global land1 3 Vol.:(0123456789)PFG surface area. (Hong et al 2016) In waters bodies, algae species play a fundamental role in water ecosystems being the primary producers in food chain and essential components in biogeochemical cycles
Satellite remote sensing has been substantial to advancing knowledge pertaining to terrestrial phenology (e.g. MacBean et al 2015; Asner et al 2000), and has increasingly been applied to marine (e.g. Arun et al 2015) and inland water (e.g. Blix et al 2018) settings using retrievals of chlorophyll-a, a common proxy for phytoplankton biomass
The merge of the Acolite calculations and the QGIS visualisation generated a chlorophyll-a map, an example is shown in Fig. 4, which illustrates the results of the four algorithms on the same day
Summary
Wetlands and rivers, surface freshwater is the most significant resource that sustains environment and the lives of the organisms. 87% of inland waters are lakes and reservoirs and cover about 1.9 million k m2 of the global land1 3 Vol.:(0123456789)PFG surface area. (Hong et al 2016) In waters bodies, algae species play a fundamental role in water ecosystems being the primary producers in food chain and essential components in biogeochemical cycles. Chlorophyll-a is the main pigment in phytoplankton, which indicates the trophic status of water (Ansper and Alikas 2019) These methods are labour intensive and costly, and do not provide synoptic views of the bloom conditions (Caballero et al 2020). The Sentinel-2 platform offers greatly improved spatial resolution over other satellite platforms designed for water based chlorophyll-a retrievals, it includes a “red-edge” band at 704 nm not present on the Landsat 8 operational land imager (Bramich et al 2021). To this aim, it is necessary to analyse the advantages and potential limitations of existing chlorophyll-a algorithms. Efforts have been made to find appropriate sensors, but none were designed for inland waters MSI (Sentinel Multispectral Imager) has appeared to be a suitable sensor for chlorophyll-a estimation due to the overall radiometric quality and temporal resolution since the launch of Sentinel-2 in 2015 (Li et al 2021)
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